Method of determining an earth-fault current

ABSTRACT

A method of determining the ground-fault current flowing in a multi-phase circuit is based on the use of standard current transformers having iron cores for the phase conductors and the neutral conductor. Allowance is made for the current-dependant linearity error of the current transformers by subtracting the product of the highest phase-current and associated error from the vectorial sum of the currents. This results in a single-sided position of the tolerance band of the measured ground-fault current and prevents spurious tripping.

BACKGROUND INFORMATION

The present invention is directed to a method of determining aground-fault current in a multi-phase system by performing a vectorialsummation of the output signals of each current transformer allocated toeach phase of the system and of a neutral conductor.

European Published Patent Application 0 407 310 B1 requires the use ofcurrent transformers having great accuracy. Spurious tripping can occurif this requirement is not satisfied. For instance, if the ground-faulttrip is to occur at 20% of the nominal current and increases by 5% whenthere is a fault in the transformer, then the measured ground current at4-times nominal phase current has a fault of 100%. According to thisreference, spurious tripping can only be avoided by employing currenttransformers that have great accuracy, but such transformers are heavyand require more space for installation. Space is limited because moderncircuit breakers also place great value on compact dimensions.

Other methods for determining a ground fault current comprise specificfeatures in terms of the requirements placed on the currenttransformers. In particular, ground currents can be measured directly,and this can be done by a ground transformer in the transformer neutralpoint or by a summation current transformer; this method is shown inU.S. Pat. No. 5,195,009. An obstacle to the high accuracy of suchmeasuring methods can be increased complexity or conflicts with thesystem configuration.

SUMMARY OF THE INVENTION

An object of the present invention is to determine a ground-faultcurrent in a multi-phase system by, first, using relatively inexpensivecurrent transformers, and second, avoiding spurious tripping in order toachieve a certain level of operational reliability in the equipment tobe protected.

This object is achieved by decreasing the vectorial sum of the currentformed from the phase currents and the current of the neutral conductorby the product of the greatest current measured at the same point intime that flows in one of the phases or in the neutral conductor and thelinearity error of the current transformer used.

This approach does not eliminate the measuring error of the groundcurrent, but rather limits its effect by forming a single-sidedtolerance band. This prevents spurious tripping caused by the measuringerrors of the current transformers. The outlay required to realize theinvention is slight in comparison to using current transformers havinggreater accuracy. The new method is equally suitable for currenttransformers having iron cores and for air-core transformers having agreat dynamic range.

The object of the present invention can be achieved advantageously usinga non-volatile memory for storing the linearity error of all currenttransformers used as a percentage. In order to achieve this object, itis not necessary to examine the linearity error of each currenttransformer individually. Rather, it is sufficient if the linearityerror of the current transformer is determined and stored as apercentage quantity with respect each type of current transformer used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of the curve of the linearity error of acurrent transformer as a function of the current level and of theassociated tolerance band.

FIG. 2 illustrates two tolerance bands of the vectorial error sum overthe primary current.

FIG. 3 illustrates a flow diagram and circuit diagram in accordance withthe present invention.

DETAILED DESCRIPTION

In FIG. 1, the straight line G represents the linear transfer of aprimary current I_(prim) into a secondary current I_(sec) as effected byan ideal current transformer. In contrast, the relationship is inreality non-linear and correlates to the curve K. This means that themeasurement values are sometimes less than and sometimes greater thanthe correct value, depending on the level of the primary currentI_(prim). The linearity error F is therefore a function of the current.This results in a tolerance band T having a percentage error F.

In FIG. 2, the vectorial sum of the errors ΣF is shown as a currentvalue over the primary current I_(prim). In this context, the toleranceband T1 correlates to a curve that occurs without special measures. Ascan be seen, it is therefore possible, based solely on the tolerance ofthe current transformer, to reach a limiting value I_(lim) of the groundcurrent that leads to tripping. By means of the method as provided inthe invention, a corrected tolerance band T2 is produced in whichspurious tripping is not possible. How the tolerance band T2 is producedis explained using FIG. 3.

Illustrated at the bottom of FIG. 3 is a circuit breaker LS, the switchcontacts of which are arranged in line with the phase conductors L1, L2,and L3 of a three-phase system. Allocated to each of these conductorsand a neutral conductor N is one current transformer W1, W2, W3, and WN.These current transformers are designed, in terms of the desired limitedstructural volume of the circuit breaker, in such a manner that errorscorrelating to the curve K in FIG. 1 can occur.

The circuit breaker LS comprises a switching mechanism, indicated by thesymbol SM, that can be actuated by an electronic trip element. Theelectronic trip element is not shown in its entirety because in thiscontext only a trigger based on a ground current is being considered.However, a microprocessor μp is indicated; it controls the processesexplained in the following.

The upper part of FIG. 3 constitutes a flowchart that illustrates theindividual steps of the method of the present invention for measuringground current. The method in accordance with the present inventionbegins when the measured values provided by the current transformers W1,W2, W3, and WN are read (step 300). In the branch on the left-hand sideof the flowchart it is shown that the error F(I_(max)) associated withthe highest current selected in step 310 from the measured values of thephase currents I_(L1), I_(L2), I_(L3), and the current in the neutralconductor I_(N) is read from an error table as a percentage value (step315). The error table is maintained in a read-only memory ROM. Anadditional branch on the right-hand side of the flowchart illustratesthe calculation of the vectorial sum of all currents measured Σ(I_(L1),I_(L2), I_(L3),I_(N)) (step 310).

Both of these above determined variables, i.e., I_(max) and Σ(I_(L1),I_(L2), I_(L3), I_(N)), are now processed in a calculation processcontrolled by the microprocessor μp in order to obtain the correctedground current I_(g) (step 320). The ground current I_(g) is determinedfrom the formula:

    I.sub.g =Σ(I.sub.L1, I.sub.L2, I.sub.L3, I.sub.N)-F*I.sub.max

Once a corrected value I_(g) of the ground current is obtained, acomparison is conducted to determine whether I_(g) is greater than orless than a limiting value I_(lim) set by the user of the circuitbreaker LS (step 325). If this condition is satisfied, themicroprocessor μp causes through the block TRIP actuation of theswitching mechanism SM and therefore opening of the switch contacts inline with the conductors L1, L2, and L3 (step 330).

In the above-described method, a single-sided tolerance band of theerror is formed when measuring the ground current. As is illustrated inFIG. 2, unless there are special measures, there is a risk that thecircuit breaker LS will be tripped, not because of the absolute level ofthe ground current, but rather because of the current-dependent error.As a result of correcting the measured value of the ground current, theerror remains below the set threshold value for such a trip, even givenhigh currents in the phase conductors.

What is claimed is:
 1. A method of determining a ground-fault current ina multi-phase system, comprising:determining a vectorial sum of aplurality of phase currents and a current of a neutral conductormeasured at a point in time, each one of the phase currents and thecurrent of the neutral conductor being associated with a correspondingone of a plurality of current transformers; determining a product of (i)a highest one of the phase currents and of the current of the neutralconductor measured at the point in time and (ii) a linearity errorassociated with the highest one of the phase currents and of the currentof the neutral conductor; and decreasing the vectorial sum by an amountequal to the determined product to determine the ground fault current.2. The method according to claim 1, wherein each current transformer isassociated with a corresponding linearity error, and wherein eachlinearity error is maintained in a non-volatile memory as a percentage.3. The method according to claim 2, wherein each linearity error isdetermined and stored as a percentage quantity as a function of anassociated one of a plurality of primary currents, each primary currentcorresponding to one of the plurality of current transformers.